sql: Print column type even if it is NULL

Some commands, e.g. EXPLAIN, invokes SEGMENTATION FAULT when being
executed through net.box. It happens due to column type of the
result of this function being NULL. This patch adds checks for
received column type.

src/box/execute.c

@@ -540,6 +540,8 @@ sql_get_description(struct sqlite3_stmt *stmt, struct obuf *out,
 		 * column_name simply returns them.
 		 */
 		assert(name != NULL);
+		if (type == NULL)
+			type = "UNKNOWN";
 		size += mp_sizeof_str(strlen(name));
 		size += mp_sizeof_str(strlen(type));
 		char *pos = (char *) obuf_alloc(out, size);

test/sql/iproto.result

@@ -773,6 +773,47 @@ cn:execute('drop table test')
 cn:close()
 ---
 ...
+--
+-- Some commands, e.g. EXPLAIN, could lead to segfault because it
+-- does not send column data type, but looks like DQL. Netbox had
+-- been trying to decode type for any DQL.
+--
+cn = remote.connect(box.cfg.listen)
+---
+...
+-- Segmentation fault when EXPLAIN executed using net.box.
+_ = cn:execute("EXPLAIN SELECT 1;")
+---
+...
+-- Segmentation fault when PRAGMA TABLE_INFO() executed using
+-- net.box.
+box.sql.execute('CREATE TABLE test (id INT PRIMARY KEY)')
+---
+...
+cn:execute("PRAGMA TABLE_INFO(test);")
+---
+- metadata:
+  - name: cid
+    type: UNKNOWN
+  - name: name
+    type: UNKNOWN
+  - name: type
+    type: UNKNOWN
+  - name: notnull
+    type: UNKNOWN
+  - name: dflt_value
+    type: UNKNOWN
+  - name: pk
+    type: UNKNOWN
+  rows:
+  - [0, 'ID', 'integer', 1, null, 1]
+...
+box.sql.execute('DROP TABLE test')
+---
+...
+cn:close()
+---
+...
 box.schema.user.revoke('guest', 'read,write,execute', 'universe')
 ---
 ...

test/sql/iproto.test.lua

@@ -247,10 +247,27 @@ cn:execute('drop table test')
 
 cn:close()
 
+--
+-- Some commands, e.g. EXPLAIN, could lead to segfault because it
+-- does not send column data type, but looks like DQL. Netbox had
+-- been trying to decode type for any DQL.
+--
+cn = remote.connect(box.cfg.listen)
+
+-- Segmentation fault when EXPLAIN executed using net.box.
+_ = cn:execute("EXPLAIN SELECT 1;")
+
+-- Segmentation fault when PRAGMA TABLE_INFO() executed using
+-- net.box.
+box.sql.execute('CREATE TABLE test (id INT PRIMARY KEY)')
+cn:execute("PRAGMA TABLE_INFO(test);")
+box.sql.execute('DROP TABLE test')
+
+cn:close()
+
 box.schema.user.revoke('guest', 'read,write,execute', 'universe')
 box.schema.user.revoke('guest', 'create', 'space')
 space = nil
 
 -- Cleanup xlog
 box.snapshot()
-